He's supposed to have been working on The Winds of Winter ever since then.
An interesting tidbit he'd mentioned before (I can't remember where I read this), was that he'd write another script for GoT maybe next year if he's done with TWOW by then, which usually happens around March-June. So, the fact that the book (or at least a draft) might be finished by March or April is entirely possible.
Either way, the release date is still likely going to be 2016, since the logistics required to release one of the most anticipated books of the year take a long time to sort out. If it releases next year, 2016 will be confirmed to be the best year for fantasy books in a long, long time with TWOW, Doors of Stone, and Stormlight Archive Book 3.